Airline travellers face further travel chaos after British Airways cabin crew rejected the airline's final pay offer in a move that raises the prospect of further strike action, which could ground flights.

The Unite union, which represents 90% of BA's 12,000 cabin crew, said 67% of its members voted against BA's final pay offer, which included two years' guaranteed basic salary increase.

The vote came as a shock to BA watchers who had expected it to be close but believed cabin crew would accept the pay deal, which offered a pay rise of 2.9% next year and 3% the year after.

The vote – with 3,419 members rejecting the deal and 1,686 accepting it – paves the way for the union to ballot for further industrial action.

Cabin crew have taken 22 days of strike action since March, costing BA more than £180m. The first new strike could come in September.

Union leaders will meet cabin crew representatives this afternoon to consider their next steps, a spokesman said.

Although Unite and its cabin crew branches, Bassa and Cabin Crew 89, agreed to take a neutral stance on the proposal, all members received a Unite summary of the offer in the form of a list of "the good, the bad, the promises and the threats".

The dispute started last year over BA's plans to cut costs by reducing the number of cabin crew on aircraft, but the row intensified after the airline withdrew travel concessions from staff who went on strike.

Relations worsened after BA took disciplinary action against union members as a result of the dispute, including sacking several staff.